U.S. Pat. No. 7,124,906 B2, issued on Oct. 24, 2006, by Hamer et al. discloses the problem of lightning strikes that hit storage vessels containing flammable materials, which can cause devastating incidents. Hamer further discloses that floating roof tanks are widely used to store volatile petroleum-based liquids and limit the quantity of product evaporative emissions that may escape to the environment. Such tanks may be configured either as internal floating-roof tanks or as external floating-roof tanks. An internal floating roof tank differs from an external roof tank in that in the former case, a separate external fixed roof is positioned above of the internal floating roof tank. In each configuration, the floating roof is designed to remain in contact with the product liquid surface and cover almost the entire surface of the product. A small annular area between the outermost rim of the floating roof and the inside surface of the tank shell is covered by a seal attached to the rim of the floating roof. According to Hamer, an effective defense against ignition by a direct stroke is a tight seal. Also, according to Hamer, fires are caused as a result of discharges caused by lightning, where the ignition can be from a direct stroke or from the sudden discharge of an induced (bound) charge on the floating roof, released when the charge on a cloud discharges to ground or to another cloud. Hamer discloses that a solution to the lightning problem is to have the floating roof electrically bonded to shoes of the seal on the circumference of the tank, but Hamer also discloses that the shunts and shoes do not adequately protect the floating roof storage tanks from the effects of a lightning strike. Hamer therefore proposes the use of a submerged bonding strap or shunt to eliminate the “above the seal” bonding strap. Therefore, Hamer clearly demonstrates that the prevailing perception of the lightning problem is limited to currents that come directly from the return stroke of the lightning discharge. This perception is repeated in the American Petroleum Institute's Recommended Practices for Lightning Protection of Aboveground Storage Tanks for Flammable of Combustible Liquids, API 545, First Edition October 2009.
However, when lightning strikes in the vicinity of a tank, whether the lightning current is conducted via a single lightning mast to ground or whether the lightning hits the ground directly, the return stroke current in either case will produce intense electromagnetic fields. These electromagnetic waves propagate radially around the lightning channel. When these waves hit an external floating roof tank, the poor electromagnetic shielding of the structure leads to induced potentials on the tank wall and on the floating roof. Wherever there are discontinuities in current paths, induced potential differences can give rise to streamer discharges or sparking. So even if the floating roof of the tank is electrically connected to the tank wall shell at a number of locations by shunts and additionally with by-pass conductors above or below the liquid level, this may not prevent streamer discharges in gaps subject to induced potentials due to the penetrating electromagnetic waves and thus fires remain a real possibility.
The application of by-pass conductors or other conductor systems that bond the wall of the tank directly to the floating roof may also further exasperate the problem by producing open conducting loops which act as magnetic loop antennas, with regards to the penetrating electromagnetic waves coming from the lightning channel.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.